When a vacuum source, such as a vacuum pump, which utilizes a lubricant, is operated to withdraw gases from a vacuum chamber, the lubricant of the pump may become vaporized and when the vacuum becomes very low and the pressure becomes approximately equal in the pump to that in the vacuum chamber, the lubricant vapor may find its way into the vacuum chamber. Lubricant vapors entering the vacuum chamber may be highly undesirable and contaminate the vacuum chamber.